Abstrict An HVAC system includes a desiccant wheel, wherein the wheel's
speed varies with airflow, the wheel is energized for at least a
set period at startup, and/or a heat recovery system (e.g., an air-to-air
heat exchanger) upstream of the wheel enhances the system's ability
to dehumidify air.
Claims What is claimed is:
1. A refrigerant system for conditioning air for a comfort zone,
the refrigerant system comprising: an enclosure defining an upstream
air passageway, a downstream air passageway, and an intermediate
air passageway therebetween, wherein the air passes sequentially
through the upstream air passageway, the intermediate air passageway,
and the downstream air passageway; a cooling coil disposed in the
enclosure; a source associated with the cooling coil and providing
a chilled fluid thereto; a blower in a position to force the air
from the downstream air passageway into the comfort zone; a desiccant
wheel able to absorb moisture from the air passing from the intermediate
air passageway to the supply air passageway and simultaneously release
moisture to the air passing from the upstream air passageway to
the intermediate air passageway; and a controller connected to selectively
start and stop the source and selectively energize and de-energize
the desiccant wheel for rotation, wherein the controller upon starting
the source energizes the desiccant wheel for a predetermined limited
period, whereby the desiccant wheel during the predetermined limited
period helps absorb moisture that may vaporize from the cooling
coil before the cooling coil is sufficiently cool to condense moisture
from the air.
2. The refrigerant system of claim 1 further comprising a moisture
sensor in communication with the controller, wherein the controller
de-energizes the desiccant wheel in response to the moisture sensor
detecting that the air is drier than a certain limit.
3. The refrigerant system of claim 1 further comprising a temperature
sensor in communication with the controller, wherein the controller
after the predetermined limited period de-energizes the desiccant
wheel in response to the temperature sensor detecting that the air
is warmer than a certain limit.
4. The refrigerant system of claim 1 wherein the source is a compressor
and the cooling coil is disposed in the upstream air passageway.
5. A refrigerant system for conditioning air for a comfort zone,
the refrigerant system comprising: an enclosure defining an upstream
air passageway, a downstream air passageway, and an intermediate
air passageway therebetween, wherein the air passes sequentially
through the upstream air passageway, the intermediate air passageway,
and the downstream air passageway; a cooling coil disposed in the
intermediate air passageway; a desiccant wheel able to absorb moisture
from the air passing from the intermediate air passageway to the
downstream air passageway and simultaneously release moisture to
the air passing from the upstream air passageway to the intermediate
air passageway; a variable air volume blower in a position to force
the air at a variable airflow rate from the downstream air passageway
to the comfort zone; and a controller connected to the variable
air volume blower to adjust the variable airflow rate and connected
to the desiccant wheel to adjust a rotational speed thereof, wherein
the controller selectively increases the rotational speed of the
desiccant wheel upon increasing the variable airflow rate and decreases
the rotational speed of the desiccant wheel upon decreasing the
variable airflow rate.
6. The refrigerant system of claim 5 wherein the rotational speed
of the desiccant wheel is proportional to the variable airflow rate
of the blower.
7. The refrigerant system of claim 5 further comprising an airflow
sensor in fluid communication with the air, wherein the variable
airflow rate of the blower is determined based on the airflow sensor.
8. The refrigerant system of claim 5 further comprising: a heater
disposed in the upstream air passageway; and a humidistat disposed
in the upstream air passageway, wherein the heater is selectively
energized and de-energized in response to the humidistat.
9. The refrigerant system of claim 5 further comprising a temperature
sensor disposed downstream of the intermediate air passageway, wherein
activation of the cooling coil is in response to the temperature
sensor.
10. The refrigerant system of claim 5 further including a source
of chilled fluid operatively associated with and connected to the
cooling coil and the controller wherein the controller upon starting
the source energizes the desiccant wheel for a predetermined limited
period, whereby the desiccant wheel during the predetermined limited
period helps absorb moisture that may vaporize from the cooling
coil before the cooling coil is sufficiently cool to condense moisture
from the air.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention generally pertains to HVAC systems and more
specifically to an air conditioning system that includes a dehumidifying
desiccant wheel.
2. Description of Related Art
Energy wheels and desiccant wheels are two distinct types of wheels
used in the HVAC industry. An energy wheel is a rotating, porous
mass that functions as heat exchanger by transferring sensible heat
from one air stream to another. With an energy wheel, half the wheel
absorbs heat while the other half releases it. Examples of energy
wheels are disclosed in U.S. Pat. Nos. 6141979 and 4825936.
Desiccant wheels, on the other hand, transfer moisture from one
air stream to another, usually for the purpose of reducing humidity
of a comfort zone. Examples of systems with desiccant wheels are
disclosed in U.S. Pat. Nos. 6311511; 6237354; 5887784; 5816065;
5732562; 5579647; 5551245; 5517828 and 4719761.
Although many air conditioning systems that are enhanced with desiccant
wheels have been developed, such systems often implement the use
of desiccant wheels whenever there is a dehumidification load. However
many air conditioning systems may be most efficient if the desiccant
wheel is only utilized at part load conditions or when the load
on the system shifts from a sensible cooling load to more of a latent
cooling or dehumidification load. Current systems often fail to
address these efficiency concerns. Moreover, current systems with
desiccant wheels often disregard a critical period when the refrigerant
system is first activated. At startup, it takes a moment for the
refrigerant system's evaporator to become sufficiently cold to remove
moisture from the air. So, when the refrigerant system is first
energized and before the evaporator becomes cold, condensed water
on the surface of the evaporator may actually evaporate into the
air, which can increase the humidity of the comfort zone.
Consequently, a need exists for air conditioning systems that are
enhanced with desiccant wheels that address efficiency concerns
at part load operation for variable air volume systems.
SUMMARY OF THE INVENTION
It is a primary object of the invention to improve an HVAC system's
overall effectiveness by configuring the system with a desiccant
wheel in a manner that takes full advantage of the wheel's ability
to reduce humidity over a variety of operating conditions.
Another object of some embodiments is to start a refrigerant compressor
and the rotation of a desiccant wheel regardless of the surrounding
humidity, and then discontinue the wheel's rotation after a predetermined
period, whereby the wheel, during the predetermined period, can
reabsorb moisture that may have vaporized off an evaporator at startup.
Another object of some embodiments is to discontinue the rotation
of a desiccant wheel in response to a humidistat indicating that
the humidity is below a certain level.
Another object of some embodiments is to discontinue the rotation
of a desiccant wheel in response to a thermostat indicating that
the air temperature is above a certain level.
Another object of some embodiments is to vary the rotational speed
of a desiccant wheel in proportion to the airflow volume through
the wheel.
Another object of some embodiments is to vary the rotational speed
of a desiccant wheel in proportion to the airflow volume through
the wheel, wherein the airflow volume is determined based on a controller's
speed command signal to a variable speed blower.
Another object of some embodiments is to vary the rotational speed
of a desiccant wheel in proportion to the airflow volume through
the wheel, wherein the airflow volume is determined based on an
airflow sensor.
Another object of some embodiments is to preheat the air entering
a desiccant wheel in response to a humidistat, wherein the preheating
assists the wheel in reducing the humidity in situations where the
rotational speed of the wheel is reduced due to lower airflow rates.
Another object of some embodiments is to heat the air entering
one portion of a desiccant wheel and cooling the air entering another
portion of the wheel, wherein the heating is in response to a humidistat,
and the cooling is in response to a temperature sensor.
Another object of some embodiments is to decrease the cooling rate
of a desiccant wheel system to meet a reduced sensible cooling demand,
while maintaining or just slightly decreasing a heating rate to
meet a latent heating demand.
Another object of some embodiments is to install a heat recovery
system upstream of a desiccant wheel to meet both a latent and sensible
cooling demand. An air-to-air heat exchanger and a condenser/evaporator
refrigerant circuit are just two examples of such a heat recovery
system.
Another object of some embodiments is to meet a latent cooling
demand without having to preheat the incoming air or otherwise increase
the sensible cooling demand.
Another object of some embodiments is to provide an HVAC enclosure
that conveys more airflow in some sections than others to accommodate
the influx of both outside air and return air.
Another object of some embodiments is to install a pre-dehumidifying
heat recovery system upstream of the desiccant wheel to meet both
a latent and sensible cooling demand.
One or more of these and/or other objects of the invention are
provided by an HVAC system that includes a desiccant wheel, wherein
the configuration and/or control of the system is such that the
system takes full advantage of the wheel's ability to cool and dehumidify
the air of a comfort zone under various conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of one embodiment of an HVAC system
that includes a desiccant wheel.
FIG. 2 is a schematic diagram of a second embodiment of an HVAC
system that includes a desiccant wheel.
FIG. 3 is a schematic diagram of a third embodiment of an HVAC
system that includes a desiccant wheel.
FIG. 4 is a schematic diagram of a fourth embodiment of an HVAC
system that includes a desiccant wheel.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A refrigerant system 10 shown in FIG. 1 is cycled on and off
to meet a latent and/or sensible cooling demand, wherein a desiccant
wheel 12 of the system operates for at least a predetermined period
at the beginning of each cycle. At the start of each cycle, it can
take a moment for a cooling coil 14 such as an evaporator of a
refrigerant circuit, to become sufficiently cool to condense moisture
from the air 16. Moisture, which may have condensed on the surface
of coil 14 during an earlier operating cycle, may later evaporate
back into the air upon starting a new cycle. So, operating wheel
12 for a predetermined period at startup can help absorb that moisture
before it raises the humidity of a comfort zone 18 such as a room
or other area of a building 20.
For the illustrated embodiment, system 10 comprises an enclosure
22 that contains cooling coil 14 desiccant wheel 12 driven by a
motor 24 a blower 26 and a controller 28.
Enclosure 22 is schematically illustrated to represent any structure
or combination of structures that can define an upstream air passageway
30 an intermediate air passageway 32 and a downstream air passageway
34. In this example, enclosure 22 comprises a cabinet 22A and a
roof curb 22B, wherein roof curb 22B attaches cabinet 22A to a roof
of building 20. Although enclosure 22 is shown having its two components,
cabinet 22A and roof curb 22B, adjacent to each other, other embodiments
may have an enclosure whose components are separated or interconnected
by ductwork.
Cooling coil 14 is schematically illustrated to represent any structure
that can cool a stream of air by means of a chilled fluid from a
chilled fluid source 33. Examples of a chilled fluid source 33 for
coil 14 include, but are not limited to, a conventional evaporator
of a conventional refrigerant circuit, and a heat exchanger that
conveys chilled water.
Blower 26 is schematically illustrated to represent any apparatus
that can move air 16 through enclosure 22. Examples of blower 26
include, but are not limited to, a centrifugal fan, an axial fan,
etc. Although blower 26 is shown disposed within intermediate air
passageway 32 blower 26 could be installed anywhere as long as
it can move air 16 in an appropriate flow path through enclosure
22.
Desiccant wheel 12 is schematically illustrated to represent any
rotatable, air-permeable structure that can absorb and release moisture
from a stream of air 16. Wheel 12 for example, may comprise a honeycomb
structure or porous pad or cage that contains or is coated with
a desiccant, such as silica gel, montmorillonite clay, zeolite,
etc. The actual structure of various desiccant wheels are well know
to those skilled in the art. Examples of desiccant wheels are disclosed
in U.S. Pat. Nos. 6311511; 6237354; 5887784; 5816065; 5732562;
5579647; 5551245; 5517828 and 4719761 all of which are
specifically incorporated by reference herein.
Controller 28 provides at least one output signal that cycles cooling
coil 14 and blower 26 on and off to meet the cooling and/or dehumidification
demand of comfort zone 18. In this example, controller 28 provides
an output signal 36 for selectively energizing or energizing the
source 33 of chilled fluid and/or the cooling coil 14 (or its associated
refrigerant compressor) and an output signal 38 for energizing blower
26. Controller 28 also provides another output signal 40 for selectively
energizing and de-energizing motor 24 of desiccant wheel 12. Controller
28 is schematically illustrated to represent any device that can
provide such output signals. Examples of controller 28 include,
but are not limited to, an electromechanical relay circuit, thermostat,
PLC (programmable logic controller), computer, microprocessor, analog/digital
circuit, and various combinations thereof.
Under normal operation, blower 26 draws return air 16A and/or outside
air 16B into intermediate air passageway 32 and across coil 14
which provides latent and sensible cooling of the air. Next, blower
26 forces the conditioned air from intermediate air passageway 32
through a portion of wheel 12 that absorbs moisture from supply
air 16C. Downstream air passageway 34 then conveys the relatively
cool, dry supply 16C to comfort zone 18. Some of the air in zone
18 may escape building 20 through a vent 42 or other outlet, and
the rest of the air becomes return air 16A that blower 26 draws
back into upstream air passageway 30. As wheel 12 rotates, wheel
12 carries the moisture it absorbed in downstream passageway 34
and releases the moisture to the return air 16A passing through
upstream air passageway 30.
Upon initially activating the source 33 and/or cooling coil 14
and blower 26 at the beginning of each on-cycle, controller 28 actuates
or rotates wheel 12 for a predetermined limited period, e.g., five
or ten minutes, regardless of any current dehumidification need.
During this period, wheel 12 can absorb moisture that the surface
of coil 14 may have accumulated from a previous on-cycle and is
currently evaporating from that surface. Such evaporation can be
caused by air 16 passing across the surface of coil 14 before the
coil is sufficiently cool to hold the moisture in a condensed state.
With wheel 12 rotating at the beginning of every on-cycle, downstream
air passageway 34 can immediately convey relatively dry supply air
16C to comfort zone 18.
Once the predetermined period expires, signal 40 can de-activate
wheel 12 while cooling coil 14 and blower 26 continue operating
to meet the sensible cooling demand of zone 18. If, however, a humidistat
44 determines that a dehumidification demand exists after the predetermined
period expires, signal 40 may command wheel 12 to continue operating.
In some cases system 10 may have difficulty meeting the sensible
cooling demand of zone 18. Such an overload can be determined based
on a thermostat 46 indicating that the zone temperature has risen
to a certain level (e.g., two degrees above a target zone temperature)
even though system 10 is still operating. In such situations, signal
40 may de-activate wheel 12 until system 10 can satisfy the zone's
sensible cooling demand.
In another embodiment, shown in FIG. 2 a refrigerant system 48
comprises desiccant wheel 12 blower 26 cooling coil 14 an optional
heater 50 and an enclosure 52. Enclosure 52 defines an upstream
air passageway 54 an intermediate air passageway 56 and a downstream
air passageway 58. Blower 26 forces air sequentially through upstream
passageway 54 through heater 50 through a first portion 12A of
wheel 12 that releases moisture to the air, into intermediate air
passageway 56 through blower 26 through cooling coil 14 to provide
latent and sensible cooling, through another portion 12B of wheel
12 to absorb moisture from the air, into downstream passageway 58
and onto a comfort zone. The air in downstream air passageway 58
is supply air, and the air in upstream air passageway 54 can be
return air and/or outside air. In this case, wheel 12 transfers
moisture from the supply air to the return air or outside air.
System 48 is particularly suited for VAV systems where the cooling
demand of a building is met by a system that delivers supply air
at a variable air volume. A controller 60 similar to controller
28 provides one or more output signals to system 48. Output signal
62 for example, controls the speed or airflow volume of blower
26 an output signal 64 controls the rotational speed of wheel 12
an output signal 66 controls cooling coil 14 (e.g., by selectively
actuating its associated compressor), and an output signal 68 controls
the operation of heater 50. To meet the building's cooling needs,
controller 60 varies the air delivery of blower 26 by providing
output signal 62 in response to an input signal 70 from a temperature
sensor 72.
To help maintain the wheel's efficiency over a range of airflow
volumes, controller 60 provides output signal 64 such that the rotational
speed of wheel 12 increases with the air volume. The wheel's speed
is preferably adjusted to be proportional to the blower's speed
or airflow volume. Controller 60 can determine the airflow volume
by way of an input signal 74 from a conventional airflow sensor
76. Alternatively, controller 60 can simply assume the airflow volume
or blower speed agrees with output signal 62 whereby flow sensor
76 can be omitted.
Heater 50 which is optional, can be used for preheating the return
air in situations where the rest of system 48 is unable to effectively
dehumidify the air without excessively cooling the supply air to
a level where the comfort zone begins feeling unpleasantly cold.
Heater 50 can be a primary or auxiliary condenser of the same refrigerant
circuit that contains cooling coil 14 or heater 50 can be a separate
heater, such as an electric heater, hot water coil, radiator, etc.
In some cases where the sensible cooling demand drops significantly
while the latent cooling demand remains high, the heat transfer
rate between heater 50 and the current of air passing therethrough
can remain constant or be reduced by a first delta-heat transfer
rate, and the heat transfer rate between cooling coil 14 and the
current of air passing therethrough can be reduced by a second delta-heat
transfer rate, wherein the second delta-heat transfer rate is greater
than the first delta-heat transfer rate. Deactivating or increasing
the surface temperature of cooling coil 14 can be the primary cause
of the second delta-heat transfer rate, while a decrease in airflow
volume can cause the first delta-heat transfer rate. If, however,
the airflow volume is not reduced, then the first delta-heat transfer
rate may be substantially zero (i.e., the heat transfer rate of
heater 68 remains substantially constant).
FIG. 3 shows a system 78 that is similar to system 48 of FIG. 2;
however, system 78 has a second cooling coil 80 and a heat recovery
system 82. With the heat recovery system and second cooling coil,
system 78 can provide greater dehumidification with little or no
auxiliary heat, i.e., heater 50 may be optional.
System 78 includes blower 26 that forces air 84 through an enclosure
86 that defines various air passageways. In some embodiments, blower
26 forces air 84 sequentially through an outside air inlet 88 a
cooling section 82A of heat recovery system 82 an intermediate
air chamber 90 cooling coil 80 a heating section 82B of heat recovery
system 82 an outside air outlet 92 an upstream air passageway
94 where return air 84A from a comfort zone and outside air 84B
can mix, optional heater 50 a moisture-releasing section 12A of
desiccant wheel 12 an intermediate air passageway 94 that contains
blower 26 and cooling coil 14 a moisture-absorbing section 12B
of wheel 12 and a downstream air passageway 96 that discharges
supply air 85C to a comfort zone.
From upstream air passageway 94 to downstream air passageway 96
the function of system 78 is very similar to that of system 48.
To enhance dehumidification, however, system 78 employs cooling
coil 80 and heat recovery system 82. Cooling coil 80 removes moisture
from the air, while heat recovery system 82 transfer heat from the
air passing from outside air inlet 88 to intermediate air chamber
90 to the air passing from intermediate air chamber 90 to outside
air outlet 92 whereby the air moving from outside air outlet 92
to upstream air passageway 94 is cooler and drier than the air entering
system 48 of FIG. 2.
The fact that the air in passageway 94 is not only drier but is
also cooler than the air in passageway 94 is an important advantage
over conventional systems that preheat or warm the air to achieve
dehumidification. With conventional systems, reheating the air increases
the sensible cooling load. With the current system, however, dehumidification
can be achieved without increasing the sensible cooling load, thus
the current system is more efficient.
Heat recovery system 82 is schematically illustrated to represent
any apparatus for transferring heat from one airstream to another.
Heat recovery system 82 for example, can be a conventional air-to-air
heat exchanger or it can be the condenser and evaporator of a conventional
refrigerant circuit.
Such a refrigerant circuit is incorporated into a system 98 that
is illustrated in FIG. 4. System 98 includes a refrigerant circuit
that comprises a refrigerant compressor 100 a condenser 102 an
expansion device 104 (e.g., a flow restriction, capillary, orifice,
expansion valve, etc.), and an evaporator 106. The refrigerant circuit
operates in a conventional manner in that compressor 100 discharges
hot pressurized refrigerant gas into condenser 102. The refrigerant
within condenser 102 condenses as the refrigerant releases heat
to the surrounding air (the air passing from an intermediate chamber
90' to an outside air outlet 92'). From condenser 102 the condensed
refrigerant cools by expansion by passing through expansion device
104. The refrigerant then enters evaporator 106 where the relatively
cool refrigerant absorbs heat from the incoming outside air. From
evaporator 106 the refrigerant returns to the inlet of compressor
100 to be compressed again. As a result, the refrigerant circuit
transfers heat from the air passing through evaporator 106 to the
air passing through condenser 102.
It should be noted, that although upstream air passageway 94 conveys
a mixture of outside air 84B and return air 84A, in some embodiments
there is no return air, only outside air. In such cases, the airflow
volume through intermediate air chamber 90 or 90' is substantially
equal to that of intermediate air passageway 94. If, however, enclosure
86 or 86' receives both outside air and return air, then intermediate
air passageway 94 conveys more air than does intermediate air chamber
90 or 90'. Any excess air can be released from the building through
some sort of exhaust or other opening in the building. |